how to plot gm in HSPICE using Expressions Builder ??

Hello everyone, I want to plot gm vs. Vgs plot for a CNFET model. I am having trouble in plotting this curve. I do get Id vs. Vgs plot but how plot gm using the expression builder ?? I mean I put this thing in the Expression blank: derivative (I(Vdd))/derivative(Vgg); Result blank: Gm.... but I get the error that "derivative must take node object" ...
Also, by directly editing the netlist as .print Gm(nFET) or .print dc gm = par('gm(nFET)' ; no plot is obtained. I use HSPICE Ver: A-2008.03 .
please help me out. :-? Thanks in advance!

hiii
u have some ways to do that ,in your hspice code u can put instruction ".probe LX7(m*)" which when u open your awanwaves it s added to your node and then u can plot it v.s. any variable u choose for your x axes,another way, u can straightlly open your awan then click on function builder then u build your structure like this "derivative('0|id(m0')/derivative('0|n0')" which id is DS current and n0 is the voltage of a dc bias voltage that u sweep on it
for example i calculate Gm for a diode connected nmos :


m0 n0 n0 gnd gnd cmosn w='1.8u' l='0.18u' m='1' geo='0'
v1 n0 gnd dc='5' ac='0'
.lib'C:\mix018_rf.l'MOS_RF

.dc sweep v1 0.1 5 0.1
.print I(m0)
.probe LX7(m0)
.op
.tran 1ns 1ms
.end
that drain and gate are connected to eachother and connect to a bias voltage v1 and source is gnd,this a an easy example,hope u get it:wink: